The present invention relates to a material for growing single crystal SiC and a method of producing single crystal SiC, and more particularly to a material for growing single crystal SiC which is used as a substrate wafer for a high temperature/high frequency semiconductor electronic element such as a light-emitting diode, an X-ray optical element, a switching element, an amplifying element, and an optical sensor, and also to a method of producing such single crystal SiC.
SiC (silicon carbide) is superior in heat resistance and mechanical strength than existing semiconductor materials such as Si (silicon) and GaAs (gallium arsenide), and has good resistance to radiation. In addition, it is easy to perform the valence control of electrons and holes by doping an impurity. Moreover, SiC has a wide band gap (for example, single crystal 6H-SiC has a band gap of about 3.0 eV, and single crystal 4H-SiC has a band gap of 3.26 eV). Therefore, it is possible to realize a large capacity, a high-frequency property, a dielectric property, and a resistance to environments which cannot be realized by the above-mentioned existing semiconductor materials. SiC receives attention and is expected as a semiconductor material for a next-generation power device.
As a method of producing (growing) single crystal SiC of this type, conventionally known are: the modified sublimation and recrystallization method (modified Lely method) in which a seed crystal is placed in a graphite crucible and the gas sublimating from SiC serving as a raw material is diffusively transported in a closed space to be recrystallized on the seed crystal that is set to a low temperature; the high-temperature epitaxial method in which epitaxial growth is conducted on a silicon substrate by using the chemical vapor deposition method (CVD method), thereby growing single crystal cubic SiC (xcex2-SiC); the magnetron sputtering method; and the plasma CVD method.
Among the above-mentioned conventional production methods, in the modified sublimation and recrystallization method, however, impurities enter during a step of growing a crystal to lower the impurity, two or more polytypes mixedly exist in the same crystal, and crystal defects are easily introduced into the interface. In addition, pin holes which are called micropipe defects, which cause a leakage current when a semiconductor device is produced, which pass through the crystal in the growing direction, and which have a diameter of several microns easily remain at about 300 to 1,000/cm2 in a grown crystal. Therefore, the method is problematic in quality.
In the high-temperature epitaxial method, the substrate temperature is so high that the amount of reevaporation is large, and it is therefore required to produce a high-purity reducing atmosphere. Consequently, the method has problems in that it is difficult to practically conduct the method from the viewpoint of installation, and that, because of epitaxial growth, the crystal growth rate is naturally limited, and hence the productivity of single crystal SiC is very poor.
In the magnetron sputtering method and the plasma CVD method, the installation is massive, and defects such as micropipe defects are inevitably produced.
As described above, in the conventional methods of producing single crystal SiC, satisfactory single crystal SiC cannot be obtained from the viewpoints of installation, and the quality due to production of micropipe defects or the like. These problems are the main cause of blocking a practical use of single crystal SiC which has superior characteristics as compared with existing semiconductor materials such as Si and GaAs as described above.
Under these circumstances, the inventors have developed and proposed a method in which a complex configured by stacking in a closely contacted state a single crystal SiC base material and a polycrystalline plate consisting of Si atoms and C atoms is heat-treated under a condition where the base material side is placed in a lower low-temperature side of a heat treatment oven, whereby polycrystal members of the polycrystalline plate are transformed in imitation of single crystal of the single crystal base material so that single crystal SiC oriented in the same direction as single crystal of the single crystal base material is integrally grown.
According to the above-mentioned method of producing single crystal SiC which has been developed and proposed by the inventors (hereinafter, referred to as the proposed production method), Si atoms and C atoms do not constitute large crystal grains between the single crystal SiC base material and the polycrystalline plate, and a heat treatment is conducted under a state where an SiC layer in which a part of or the substantial whole of the Si and C atoms exist as a single member is interposed. Therefore, Si atoms and C atoms diffusively move in an interface between the single crystal SiC base material and the polycrystalline plate, and a single crystal can be integrally grown by solid-phase growth in which the diffusively moving Si and C atoms are simultaneously rearranged over a substantially whole area of the surface of the single crystal SiC base material which is on the low-temperature side, in imitation of single crystal of the single crystal SiC base material. Furthermore, impurities are prevented from entering from the atmosphere to the interface, whereby occurrence of crystal defects and distortion can be suppressed, and also occurrence of micropipe defects can be reduced. Therefore, the proposed production method has an advantage that single crystal SiC of a high quality can be grown with high productivity as compared with the above-mentioned conventional production methods.
The above-mentioned proposed production method has been further investigated. Finally, it has been found that, in order to grow single crystal SiC of a higher quality with higher productivity, the followings are important: to uniformly maintain the closely contacted state between the single crystal SiC base material and the polycrystalline plate, over the whole face; not to apply distortion on single crystal of the base material; and to uniformly maintain the temperature difference between opposing faces of the single crystal SiC base material and the polycrystalline plate over the whole region during heat treatment, to expedite lattice rearrangement of Si and C atoms which sublimate from the polycrystalline plate, in the surface of the base material in imitation of the underground single crystal SiC. The proposed production method has room for improvement in these points.
The invention has been conducted in view of the above-mentioned circumstances of the prior art. It is an object of the invention to provide a material for growing single crystal SiC that enables single crystal SiC of a very high quality in which micropipe defects and the like hardly occur, and a method of producing single crystal SiC which can produce single crystal SiC of a high quality easily and with high productivity from the viewpoints of installation and workability, thereby expediting practical use of single crystal SiC as a semiconductor material.
The material for growing single crystal SiC according to the first invention is characterized in that the base material is configured by a complex formed by stacking in a closely contacted state a single crystal SiC base material and a polycrystalline plate consisting of Si atoms and C atoms with interposing an organic or inorganic substance between opposing faces of the two members in a laminated manner, fundamental components of the substance being Si and O, and the complex is heat-treated to cause polycrystal members of the polycrystalline plate to be transformed in a same direction as single crystal of the single crystal SiC base material, thereby enabling single crystal to be grown. The method of producing single crystal SiC according to the second invention is characterized in that a single crystal SiC base material and a polycrystalline plate consisting of Si atoms and C atoms are stacked in a closely contacted state with interposing an organic or inorganic substance between opposing faces of the two members in a laminated manner, fundamental components of the substance being Si and O, and the complex is then heat-treated to cause polycrystal members of the polycrystalline plate to be transformed in a same direction as single crystal of the SiC single crystal base material, thereby integrally growing single crystal.
According to the first and second inventions having such constituents, since the organic or inorganic substance containing Si and O as fundamental components is interposed in a laminated manner between the opposing faces of the single crystal SiC base material and the polycrystalline plate, (a) the closely contacted state of the opposing faces can be uniformly held over the whole face without producing a gap between the opposing faces of the base material and the polycrystalline plate. (b) The interposed organic or inorganic substance forms an amorphous (glassy) layer of a low thermal conductivity, and the temperature difference between the opposing faces of the base material and the polycrystalline plate can be maintained substantially uniformly over the whole region during heat treatment under a state where the base material side is lower in temperature. (c) With the advance of the heat treatment, Si and O which are the fundamental components of the intermediate layer are thermally decomposed, and the intermediate layer enters a mixed state in which Si seeds and arbitrary O seeds (for example, radicals, ions, and the like) are contained. In this state, the atomic bonding between an Si atom and a C atom in the contacting face on the side of the polycrystalline plate is weakened particularly by the electron withdrawing function of the O seeds, and the stereoscopic function in which, for example, the O seeds enter Si-C lattices to produce distortion in the lattice interval. Then, Si atoms and C atoms which are produced as a result of breakage of the bond in the contacting face portion rapidly diffusively move in the intermediate layer having a temperature gradient of 100xc2x0 C. or larger. At the instance when the atoms reach the surface of the base material which is held to a low temperature and stable, lattice rearrangement of Si and C atoms in imitation of single crystal of the base material is caused so that signal crystals can be integrally grown. (d) Since the closely contacted state of the opposing faces of the base material and the polycrystalline plate is uniformly held over the whole face, it is possible to prevent impurities from entering from the atmosphere to the interface during heat treatment. (e) Even during heat treatment, the temperature of the base material is held to a low level, and hence single crystal of the base material can be stabilized.
As described above, when the organic or inorganic substance containing Si and O as fundamental components is interposed between the opposing faces of the single crystal SiC base material and the polycrystalline plate, the functions of (a) to (e) above can be attained. By a synergistic effect of these functions, even when only means for heat-treating the complex which is simple from the viewpoints of installation and workability is provided, single crystal SiC of a very high quality in which crystal defects and distortion of the base material can be prevented from occurring and micropipe defects hardly occur can be obtained with high productivity. As a result, it is possible to attain an effect that the practical use of single crystal SiC which is superior in a high temperature property, a high frequency property, a dielectric property, and a resistance to environments to existing semiconductor materials such as Si (silicon) and GaAs (gallium arsenide) and which is expected as a semiconductor material for a power device is enabled.
In the material for growing single crystal SiC according to the first invention and the method of producing single crystal SiC according to the second invention, preferably, single crystal xcex1-SiC is used as the single crystal SiC base material. In consideration of further enhancement of the quality of single crystal SiC, it is preferable to use a member which is produced into a plate-like shape by the thermochemical vapor deposition method (hereinafter, referred to as the thermal CVD method) in which impurities enter at the lowest degree is used as the polycrystalline plate. Alternatively, amorphous SiC or a high-purity SiC sintered member may be used.
In the material for growing single crystal SiC according to the first invention and the method of producing single crystal SiC according to the second invention, the intermediate layer may be interposed in a liquid phase state, whereby it is possible to completely eliminate formation of a gap between the opposing faces of the base material and the polycrystalline plate, so as to ensure the closely contacted state of the opposing faces.
In the material for growing single crystal SiC according to the first invention and the method of producing single crystal SiC according to the second invention, when silicon or a silicon compound including silicon rubber, silicon resin, silicon oil, and silicon grease is used as the organic or inorganic substance forming the intermediate layer, the intermediate layer can be formed by simple means such as application, so that the base material configured by the complex can be easily produced and the production cost can be reduced. Alternatively, the single crystal SiC base material or the polycrystalline plate itself may be thermally oxidized to form an SiO2 film in the surface, and the film may be used as the intermediate layer.
In the method of producing single crystal SiC according to the second invention, a complex in which the polycrystalline plate is formed to be larger than the single crystal SiC base material may be used, and the complex may be heat treated while the complex is placed on a support table in a heat treatment oven so that a portion which is on the side of the single crystal SiC base material and which is smaller is positioned in a lower side. According to this configuration, the polycrystalline plate which is larger easily receives radiation heat in the oven, a temperature difference is easily produced, and the heat distribution over the whole area of the interface is easily uniformalized. Therefore, the growing rate of the single crystal SiC can be increased, so that the productivity can be further improved.
When a graphite sheet is laid on a face of the support table in the heat treatment oven, the single crystal SiC base material being to be placed on the face, the support table made of sintered graphite and the single crystal SiC base material can be prevented from sticking together, and heat can be uniformly applied over the whole area of the single crystal SiC base material.